In LTE-Advanced (Release 11), which is an evolved version of 3GPP LTE (3rd Generation Partnership Project Long-term Evolution, hereinafter referred to as “LTE”), heterogeneous network (HetNet) is under study, which uses a plurality of base stations providing coverage areas in different sizes, for further capacity improvement.
HetNet is a network that uses a macro base station, which provides a large coverage area, in combination with a pico base station, which provides a small coverage area. The macro base station may also be called “macro cell,” “HPN (High Power Node)” or “macro eNB.” The pico base station may also be called “pico cell,” “LPN (Low Power Node),” “low power RRH (Remote Radio Head)” or “pico eNB.”
In LTE-Advanced, studies are being carried out on operation of coordinated transmission/reception by a plurality of base stations (CoMP: coordinated multiple point transmission and reception) in a HetNet environment. CoMP is a communication scheme mainly intended to improve the throughput of a ten Anal (UE) located at a cell edge in which a plurality of base stations (cells) cooperate to transmit and receive signals to and from the terminal.
In the case of uplink CoMP (hereinafter referred to as “UL CoMP”), a plurality of base stations (cells or reception points (RPs) cooperate to receive uplink signals (uplink signals) transmitted from one terminal. Received signals are combined by the plurality of base stations, and receiving quality is thereby improved.
Next, transmission power control of an uplink data signal (PUSCH (Physical Uplink Shared Channel), uplink data) in conventional (Release 10) LTE-Advanced will be described.
For example, transmission power PPUSCH,c(i) of PUSCH in subframe #i of serving cell #c is calculated according to following equation 1 (e.g., see NPL 1). The “serving cell” refers to a base station (cell) that indicates control information to the terminal. A downlink channel is used to indicate control information. The terminal measures reception levels (RSRP: Reference Signal Received Power) of downlink reference signals transmitted from neighboring base stations, and a base station (cell) that corresponds to the highest RSRP becomes a serving cell for the terminal.
                    (                  Equation          ⁢                                          ⁢          1                )                                                                                  P                          PUSCH              ,              c                                ⁡                      (            i            )                          =                  min          ⁢                      {                                                                                                                              P                                                  CMAX                          ,                          c                                                                    ⁡                                              (                        i                        )                                                              ,                                                                                                                                                                                                                        10                            ⁢                                                                                                                  ⁢                                                                                          log                                10                                                            ⁡                                                              (                                                                                                      M                                                                          PUSCH                                      ,                                      c                                                                                                        ⁡                                                                      (                                    i                                    )                                                                                                  )                                                                                                              +                                                                                    P                                                              O_PUSCH                                ,                                c                                                                                      ⁢                                                          (                              j                              )                                                                                +                                                                                                                                                                                                                                                        α                                                                  c                                  ⁢                                                                                                                                                                                                    ⁡                                                              (                                j                                )                                                                                      ·                                                          PL                              c                                                                                +                                                                                    Δ                                                              TF                                ,                                c                                                                                      ⁡                                                          (                              i                              )                                                                                +                                                                                    f                              c                                                        ⁡                                                          (                              i                              )                                                                                                                                                                                                }                                              [        1        ]            
In equation 1, PCMAX,c(i) [dBm] represents the maximum transmission power of PUSCH that can be transmitted from the terminal, MPUSCH,c(i) represents the number of frequency resource blocks allocated to PUSCH, Po_PUSCH,c(j) [dBm] represents a target value (parameter set by the base station) of PUSCH transmission power, PLc represents a path loss level [dB] measured by the terminal, αc(j) represents a weighting factor indicating a compensation ratio of path loss (PLc) (a parameter set from the base station {0, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1}), ΔTF,c(i) represents an offset value dependent on MCS of PUSCH, fc(i) represents a cumulative value in subframe #i including past values of a TPC (Transmission Power Control) command to be subjected to closed-loop control (control value, e.g., +3 dB, +1 dB, 0 dB, −1 dB).
Po_PUSCH,c(j) shown in equation 1 is an additional value of two parameters: Po_NOMINAL_PUSCH,c(j) and Po_UE_PUSCH,c(j). Po_NOMINAL_PUSCH,c(j) is a cell-specific parameter (value set for each cell, value used commonly by all terminals in the same cell) which is indicated with a step width of 1 [dB] over a range of −126 to 24 [dBm]. On the other hand, Po_UE_PUSCH,c(j) is a terminal-specific parameter (value set for each terminal) which is indicated with a step width of 1 [dB] over a range of −8 to 7 [dBm]. For example, as shown in FIG. 1, Po_UE_PUSCH,c(j) represented by a bit string of 4 bits (simply expressed as “Po_UE_PUSCH” in FIG. 1, and the same shall apply hereinafter) is indicated from the base station to the terminal (e.g., see NPL 1).
Values are set for Po_PUSCH,c(j) and αc(j) shown in equation 1 in correspondence with j=0, 1 and 2 respectively according to the type of transmission data. Types of transmission data are, for example, PUSCH transmission to which dynamic scheduling is applied, PUSCH transmission to which semi-persistent scheduling is applied or PUSCH transmission for RACH response.
In the aforementioned transmission power control of PUSCH, when CoMP operation in a HetNet environment is taken into consideration, interference of an uplink signal (hereinafter referred to as “uplink interference”) from a terminal (hereinafter referred to as “macro terminal (macro UE)”) controlled by a macro base station (macro eNB) to a terminal (hereinafter referred to as “pico terminal (pico UE)”) controlled by a pico base station (pico eNB) poses a problem.
FIG. 2 illustrates an example of uplink interference in a HetNet environment.
Power for compensating for path loss between macro UE and macro eNB which is a serving cell is set for uplink transmission power of macro UE shown in FIG. 2. On the other hand, power for compensating for path loss between pico UE and pico eNB is set for uplink transmission power of pico UE shown in FIG. 2. Here, as shown in FIG. 2, when macro UE is located in a region near a cell edge of the macro cell (hereinafter referred to as “cell edge region”) or in a place where it is difficult to receive a direct wave from macro eNB (e.g., behind an obstacle such as a building), path loss between macro UE and macro eNB increases. In this case, uplink transmission power set in macro UE is assumed to be greater than uplink transmission power set in pico UE. Thus, in such a situation, an uplink signal transmitted from macro UE may provide uplink interference to an uplink signal transmitted from pico UE. Particularly, as shown in FIG. 2, when macro UE is located near pico cell, the influence of uplink interference becomes greater.
In order to solve the uplink interference problem in a HetNet environment, studies are being carried out on setting of uplink transmission power (power for compensating for path loss with the reception point) intended for a reception point (base station) with a minimum path loss among a plurality of reception points for terminals to which UL CoMP is applied (hereinafter referred to as “CoMP UE”). For example, in FIG. 2, when macro eNB and pico eNB cooperate to receive an uplink signal of macro UE, power for compensating for path loss between pico UE and macro UE is set for uplink transmission power of macro UE (CoMP UE). Thus, since it is possible to reduce an interference level from CoMP UE to pico cell, a system performance improvement effect using UL CoMP can be expected.
When the conventional uplink transmission power for a serving cell is assumed to be given by equation 1, the uplink transmission power intended for a reception point corresponding to a minimum path loss is expressed by equation 2.
                    (                  Equation          ⁢                                          ⁢          2                )                                                                                  P                          PUSCH              ,              c                                ⁡                      (            i            )                          =                  min          ⁢                      {                                                                                                                              P                                                  CMAX                          ,                          c                                                                    ⁡                                              (                        i                        )                                                              ,                                                                                                                                                                                                                        10                            ⁢                                                                                                                  ⁢                                                                                          log                                10                                                            ⁡                                                              (                                                                                                      M                                                                          PUSCH                                      ,                                      c                                                                                                        ⁡                                                                      (                                    i                                    )                                                                                                  )                                                                                                              +                                                                                    P                                                              O_PUSCH                                ,                                c                                                                                      ⁢                                                          (                              j                              )                                                                                +                                                                                                                                                                                                                                                        α                                                                  c                                  ⁢                                                                                                                                                                                                    ⁡                                                              (                                j                                )                                                                                      ·                                                          (                                                                                                PL                                  c                                                                +                                                                  Δ                                  PL                                                                                            )                                                                                +                                                                                    Δ                                                              TF                                ,                                c                                                                                      ⁡                                                          (                              i                              )                                                                                +                                                                                    f                              c                                                        ⁡                                                          (                              i                              )                                                                                                                                                                                                }                                              [        2        ]            
In addition to the parameter in equation 1, ΔPL is set in equation 2. Moreover, ΔPL represents a difference between a minimum path loss level among path loss levels between the terminal and a plurality of reception points, and a path loss level between the terminal and the serving cell (hereinafter referred to as “path loss difference”). For example, ΔPL set in macro UE shown in FIG. 2 becomes a difference between the path loss level between macro eNB and macro UE, and the path loss level between pico eNB and macro UE.
Path loss difference ΔPL depends on a transmission power difference between macro eNB and pico eNB. For example, NPL 2 discloses that path loss difference ΔPL can take values in a range of 0 to −16 [dB]. NPL 3 proposes to newly add a terminal-specific parameter (e.g., path loss difference ΔPL shown in equation 2) to correct transmission power for CoMP UE in addition to Po_UE_PUSCH,c(j).